Fresnal distance is tolerable to error within reason. what happens is range is effected. a Properly alligned Pix board will get you over 100 feet has been my experience. so varying the .65 or 5/8" distance will decrease this range.
I wouldn't worry about it too much. I use RS sensors but the pir is probably the same. I set mine at 3/4 to 7/8 inch on the 380 and owl's but in the case for the 32's and 41's I have it set on 1/2 inch and works great. I'm like you in that I set them on trails only (12 to 20 feet) and they work great.